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    <title>Document</title>
    <script>
        /*
          load:加载页面的函数。页面：h5,css,图片
           DOMContentLoaded:页面重新加载，不包含图片 css
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    //    btn.addEventListener('click',function(){
    //        console.log(123);
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    //        console.log('99999');
    //        var btn = document.querySelector('button');
    //        btn.addEventListener('click',function(){
    //            console.log(123);
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        var btn = document.querySelector('button');
        btn.addEventListener('click',function(){
            console.log(123);
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